Modern technology makes it possible to track head and/or eye movement. Head movement is usually tracked by a special marker device that a user has to wear. Multiple video cameras and/or accelerometers interpret head movements and move a pointer accordingly like a mouse device.
Eye movement tracking is done by an infrared LED (light emitting diode) shining a weak beam on the eyeball. The reflection of the beam is at a fixed position relative to the tracking device. When the eye moves, the relative positions of the pupil and the beam reflection changes. This change is tracked by a video camera and interpreted as point movements on the screen.
Head and eye tracking devices are useful for those with reasonable eye sight but limited dexterity or muscle control to move a regular mouse or track ball.